money.compare
Compare two amounts of one currency, refusing to compare across currencies; min, max and clamp too.
2.0.0 · published 2026-10-03 by charlie · Anterra
Pinned by 39 tests, run in TypeScript, Python and Rust.compareMoney 11 · minMoney 9 · maxMoney 8 · clampMoney 11
What it does
`compareMoney(a, b)` is -1, 0 or 1, the shape every sort function takes. It is exactly -1, 0 or 1, not the difference `a - b`, so a caller can switch on it and it cannot overflow.
Comparing GBP 10 with EUR 10 is an error ("currency mismatch"), not an answer. Which of the two is larger depends on an exchange rate, and a comparison that quietly looks only at the numbers will sort a mixed-currency list into nonsense. Convert first with `money.convert`.
The functions
A group: 4 functions that work together, each in its own file, each pinned by its own tests in TypeScript, Python and Rust. A project can install only the ones it calls.
- compare_money (a: Money, b: Money) -> int
- min_money (a: Money, b: Money) -> Money
- max_money (a: Money, b: Money) -> Money
- clamp_money (amount: Money, min: Money, max: Money) -> Money
Once installed, your code imports each one from the group's module.
compare_money throws on bad input 11 tests
def compare_money(a: Money, b: Money) -> int| a | Money | |
| b | Money | |
| returns | int | -1 if a is less than b, 0 if equal, 1 if greater |
For example
compare_money(£1.00, £2.00)→ -1 less is -1compare_money(£2.00, £1.00)→ 1 greater is 1compare_money(£19.99, £19.99)→ 0 equal is 0
from fune.money.compare import compare_money # money.compare@^2Imports name this capability’s declared dependencies, which fune builds next to it in your project; each one links to its page.
from .money_amount import Money, assert_same_currency ← from money.amount ^1.0.0 · built alongside by fune
def compare_money(a: Money, b: Money) -> int:
"""-1, 0 or 1, comparing two amounts of the same currency.
Refuses mixed currencies: which of GBP 10 and EUR 10 is larger depends on a
rate, and a comparison that ignores the currency sorts a list into nonsense.
"""
assert_same_currency(a, b)
return -1 if a.minor < b.minor else 1 if a.minor > b.minor else 0min_money throws on bad input 9 tests
def min_money(a: Money, b: Money) -> Money| a | Money | |
| b | Money | |
| returns | Money | the smaller amount; a when they are equal |
For example
min_money(£5.00, £3.00)→ £3.00 the smaller of two amountsmin_money(£3.00, £5.00)→ £3.00 order does not mattermin_money(£19.99, £19.99)→ £19.99 equal amounts give that amount
from fune.money.compare import min_money # money.compare@^2Imports name this capability’s declared dependencies, which fune builds next to it in your project; each one links to its page.
from .money_amount import Money ← from money.amount ^1.0.0 · built alongside by fune
from .money_compare_compare_money import compare_money ← compareMoney, another function of this group · built into the same file, even by a slim install
def min_money(a: Money, b: Money) -> Money:
"""The smaller amount; the first when they are equal."""
return b if compare_money(b, a) < 0 else amax_money throws on bad input 8 tests
def max_money(a: Money, b: Money) -> Money| a | Money | |
| b | Money | |
| returns | Money | the larger amount; a when they are equal |
For example
max_money(£5.00, £3.00)→ £5.00 the larger of two amountsmax_money(£3.00, £5.00)→ £5.00 order does not mattermax_money(£19.99, £19.99)→ £19.99 equal amounts give that amount
from fune.money.compare import max_money # money.compare@^2Imports name this capability’s declared dependencies, which fune builds next to it in your project; each one links to its page.
from .money_amount import Money ← from money.amount ^1.0.0 · built alongside by fune
from .money_compare_compare_money import compare_money ← compareMoney, another function of this group · built into the same file, even by a slim install
def max_money(a: Money, b: Money) -> Money:
"""The larger amount; the first when they are equal."""
return b if compare_money(b, a) > 0 else aclamp_money throws on bad input 11 tests
def clamp_money(amount: Money, min: Money, max: Money) -> Money| amount | Money | |
| min | Money | lowest amount allowed, inclusive |
| max | Money | highest amount allowed, inclusive; must not be below min |
| returns | Money |
For example
clamp_money(£7.50, £5.00, £10.00)→ £7.50 inside the range it is unchangedclamp_money(£1.00, £5.00, £10.00)→ £5.00 below the range it is lifted to minclamp_money(£25.00, £5.00, £10.00)→ £10.00 above the range it is capped at max
from fune.money.compare import clamp_money # money.compare@^2Imports name this capability’s declared dependencies, which fune builds next to it in your project; each one links to its page.
from .math_clamp import clamp ← from math.clamp ^1.0.0 · built alongside by fune
from .money_amount import Money, assert_same_currency, money ← from money.amount ^1.0.0 · built alongside by fune
def clamp_money(amount: Money, min: Money, max: Money) -> Money: # noqa: A002 - the manifest names them min and max
"""``amount`` held within ``min..max``, all three in one currency.
Bounds the wrong way round are an error (from math.clamp), not an answer:
capping a refund at the original charge with the bounds swapped is a bug
to surface.
"""
assert_same_currency(amount, min)
assert_same_currency(amount, max)
return money(clamp(amount.minor, min.minor, max.minor), amount.currency)Install
fune buildWith that line in your source, in a Python project (language python in fune.project), fune build resolves it and its 2 dependencies, pins them in fune.lock, downloads only the Python package of each, and builds the code above into your project’s .fune/build, one readable file per capability with a header linking back here. Or pin a range in fune.project and build in one step:
fune add money.compareThat builds the whole group. To build only what you call, and whatever it uses inside the group:
fune add money.compare --only compareMoneyThe manifest, vectors and README with only the Python implementation. Install it without the registry with fune add ./money.compare-2.0.0-python.fune, or fetch it from a terminal with fune pull money.compare@2.0.0:python.
The whole function, every language, is one file too: money.compare-2.0.0.fune, 21,448 bytes, sha256 be921740d107bfb3f817355a1724582ab8bf91eb38f3977ac0a346c71dfcc541. It installs into a project of any language.
Customise it in your app
The seams this capability offers. Put a marker directly above a function of your own and fune build wires it into the built code; the package on the registry is not changed, the built file’s header lists it under CUSTOMISED, and fune hooks lists every hook in the project. How hooks work.
before — your function gets the arguments and returns them, changed or not, or throws to refuse the call.
# fune: before money.compare.compareMoney
# fune: before money.compare.minMoney
# fune: before money.compare.maxMoney
# fune: before money.compare.clampMoneyafter — your function gets the result and the arguments, and returns the final result.
# fune: after money.compare.compareMoney
# fune: after money.compare.minMoney
# fune: after money.compare.maxMoney
# fune: after money.compare.clampMoneyreplace — inside this capability’s code only, calls to a dependency go to your function, with the same signature. Other capabilities that use it are unaffected; write in * to replace it everywhere.
# fune: replace math.clamp in money.compare
# fune: replace money.amount in money.comparestep — your function runs at a numbered point inside a function’s body, receives the in-scope values it names as parameters, and may return replacements. List the points with fune show money.compare --steps.
# fune: step money.compare.<fn> after <n|label>Tests
A version published now needs at least 8 tests for every function, and one that expects the error for each function that throws; the registry refuses it otherwise. fune verify --all runs each case in TypeScript, Python and Rust, and a project runs them again with fune verify. This page lists the cases; it does not run them. The exact JSON is vectors.json.
compareMoney 11 tests
| Case | Arguments | Expected | |
|---|---|---|---|
| less is -1 | £1.00, £2.00 | → | -1 |
| greater is 1 | £2.00, £1.00 | → | 1 |
| equal is 0 | £19.99, £19.99 | → | 0 |
| the answer is 1, not the difference of 1998 | £19.99, £0.01 | → | 1 |
| a larger debt is less | -£5.00, -£1.00 | → | -1 |
| zero is more than a credit | £0.00, -£0.01 | → | 1 |
| one minor unit apart | £100.00, £100.01 | → | -1 |
| yen compares in whole yen | ¥1,500, ¥1,499 | → | 1 |
| the largest safe amounts still compare exactly | £90,071,992,547,409.91, £90,071,992,547,409.90 | → | 1 |
| opposite extremes | -£90,071,992,547,409.91, £90,071,992,547,409.91 | → | -1 |
Show the other 1 test
| Case | Arguments | Expected | |
|---|---|---|---|
| comparing across currencies is an error, not an answer | £10.00, €10.00 | → | error: currency mismatch |
minMoney 9 tests
| Case | Arguments | Expected | |
|---|---|---|---|
| the smaller of two amounts | £5.00, £3.00 | → | £3.00 |
| order does not matter | £3.00, £5.00 | → | £3.00 |
| equal amounts give that amount | £19.99, £19.99 | → | £19.99 |
| a refund is smaller than a charge | -£2.50, £1.00 | → | -£2.50 |
| the larger debt is the smaller amount | -£5.00, -£1.00 | → | -£5.00 |
| zero against a one penny credit | £0.00, -£0.01 | → | -£0.01 |
| yen compare in whole yen | ¥1,500, ¥1,499 | → | ¥1,499 |
| opposite extremes of the safe range | £90,071,992,547,409.91, -£90,071,992,547,409.91 | → | -£90,071,992,547,409.91 |
| mixed currencies are an error, not an answer | £1.00, €2.00 | → | error: currency mismatch |
maxMoney 8 tests
| Case | Arguments | Expected | |
|---|---|---|---|
| the larger of two amounts | £5.00, £3.00 | → | £5.00 |
| order does not matter | £3.00, £5.00 | → | £5.00 |
| equal amounts give that amount | £19.99, £19.99 | → | £19.99 |
| a charge is larger than a refund | -£2.50, £1.00 | → | £1.00 |
| the smaller debt is the larger amount | -£5.00, -£1.00 | → | -£1.00 |
| zero against a one penny credit | £0.00, -£0.01 | → | £0.00 |
| one minor unit apart at the top of the safe range | £90,071,992,547,409.90, £90,071,992,547,409.91 | → | £90,071,992,547,409.91 |
| mixed currencies are an error, not an answer | $1.00, £1.00 | → | error: currency mismatch |
clampMoney 11 tests
| Case | Arguments | Expected | |
|---|---|---|---|
| inside the range it is unchanged | £7.50, £5.00, £10.00 | → | £7.50 |
| below the range it is lifted to min | £1.00, £5.00, £10.00 | → | £5.00 |
| above the range it is capped at max | £25.00, £5.00, £10.00 | → | £10.00 |
| exactly min is kept | £5.00, £5.00, £10.00 | → | £5.00 |
| exactly max is kept | £10.00, £5.00, £10.00 | → | £10.00 |
| a refund is capped at the original charge, as a negative range | -£15.00, -£10.00, £0.00 | → | -£10.00 |
| zero-width range pins the amount | £0.42, £0.00, £0.00 | → | £0.00 |
| a range entirely of credits | -£0.10, -£5.00, -£1.00 | → | -£1.00 |
| bounds the wrong way round are an error | £7.50, £10.00, £5.00 | → | error: min must not be greater than max |
| a min in another currency is an error | £7.50, €5.00, £10.00 | → | error: currency mismatch |
Show the other 1 test
| Case | Arguments | Expected | |
|---|---|---|---|
| a max in another currency is an error | £7.50, £5.00, $10.00 | → | error: currency mismatch |
More from the author
`minMoney(a, b)` and `maxMoney(a, b)` are built on `compareMoney`, with the same currency check, and return the first argument when the two are equal. `clampMoney(amount, min, max)` holds an amount within an inclusive range, all three in one currency, using `math.clamp`. It refuses a range whose `min` is above its `max` ("min must not be greater than max"): capping a refund at the original charge, or a discount at the order total, with the bounds the wrong way round is a bug to surface, not a value to return.
This is a group of four functions, each in its own file. `minMoney` and `maxMoney` import `compareMoney`, so installing either with `only=` brings it too; `clampMoney` stands alone.
## What changed from 1.0.0
1.0.0 was one function, `compareMoney`, with `minMoney`, `maxMoney` and `clampMoney` exported beside it but unpinned: no signature in the manifest and no vectors, so nothing held the three languages to the same answer for them. 2.0.0 is a group in which all four are published functions with their own signatures and vectors. `compareMoney` keeps every 1.0.0 vector.
No answer changed. It is a new major version because the package's shape changed: it installs as one module per function plus the group module (`money_compare` still re-exports all four), and a project can take only some of it. Dependents stay on `^1.0.0` until they move deliberately; 1.0.0 is unchanged.